In the operation of nuclear power stations, nuclear fuel encased in metal tubes, known as fuel elements, are burned up inside the nuclear reactor core. Following reactor discharge, the resulting spent fuel elements contain radioactive material which decays, generating large quantities of heat and radiation emissions. The industry practice is to place the spent fuel in water pools located on the station site, given that water has excellent heat removal properties and provides adequate radiation shielding. Long term storage of spent fuel in water pools produces large quantities of loose radioactive material contamination, which remains suspended in the water and deposited on the water pool floor.
Depending on the concentration of the uranium in the fuel and the level of burn-up, the spent fuel elements must remain in water pools for a period varying from 5-10 years, at which point the heat generation is reduced to a level where the fuel elements can be removed and placed in a dry shielded storage facility. The spent fuel is isolated and the radiation emissions must be shielded for a period of several thousand years. Future final disposal is expected to take place once a final underground repository has been located and becomes operable.
Over the years, dry shielded storage technologies have been developed, including containers having a core of steel and lead, such as those in U.S. Pat. No. 3,229,096 to Bonilla et al; U.S. Pat. No. 2,514,909 to Strickland; and U.S. Pat. No. 4,666,659 to Lusk et al. Lead and steel have excellent shielding and heat transfer capabilities. Large quantities of spent fuel can be stored per container while dissipating sufficient heat to ensure that the structural material properties are maintained within acceptable standards and codes.
Containers utilizing steel and lead shielding materials also allow for safe and simple handling. The steel outer surfaces of these containers enable them to be lowered directly into water storage pools for spent fuel loading. The surface material characteristics have been designed to have low surface porosity. The spent fuel elements are transferred directly into the steel and lead containers. The steel and lead shielded lid is fastened onto the container below the surface of the water prior to removing the container from the water storage pool. Underwater fuel loading reduces direct radiation exposure to nuclear power plant workers.
However, prior to transferring the steel and lead containers to the outdoor storage site, large quantities of loose radioactive contamination must be chemically removed from the container outer surface in order to comply with applicable nuclear standards and codes. Thus, storage containers using steel and lead suffer from a significant technical disadvantage, since the chemical decontamination of the outer metal surface exposes the nuclear power plant workers to significant toxic chemicals.
Storage containers using steel and lead also suffer from a significant economic disadvantage, resulting from the high cost of material and fabrication. Since nuclear power plants are now transferring from water storage pools large volumes of spent fuel, the cost of this "old" technology is seen as prohibitive and new lower cost technology has been developed.
In particular, a number of different types of concrete containers have been developed. One concrete container utilizes an inner and outer steel shell and an inner concrete shield. Concrete and steel as a storage container has an economic advantage due to the low cost of material and fabrication relative to lead and steel.
A permanent metal-clad concrete container is described in U.S. Pat. No. 5,102,615 to Armstrong and Grande et al for pool loading applications, and a concrete cask storage system is described in U.S. Pat. No. 4,800,062 to Craig, Haelsig, Kent, Harbor and Schmoker et al for on-site spent fuel storage.
Concrete material as a radiation shield has a technical advantage since it has a high water content, and water is an excellent radiation shield. A further technical advantage of a concrete container with a steel outer surface is that spent fuel elements can be loaded below water in the water storage pools in a similar manner to the steel and lead storage containers.
However, concrete shielded containers which utilize permanent inner and outer steel liners and a concrete inner shield suffer from significant technical disadvantages. The chemical decontamination of the outer metal surface exposes the nuclear power plant workers to significant toxic chemicals. Also, since concrete has poor heat transfer characteristics, these concrete shielded containers tend to exceed acceptable standards and codes when relatively low quantities of heat is generated. Most commercial applications require storage quantities of spent fuel and associated levels of heat generation which exceed the limits of this technology.
With a view to overcoming these limitations, concrete containers employing heat removal systems have been developed. In order to allow for the maximum removal of heat generated from radioactive decay, the outer metal surfaces have been eliminated and air ventilation penetrations to the inner steel liner have been added to permit heat removal. With the addition of the heat removal systems, large quantities of spent fuel can be stored with concrete containers.
Concrete containers utilizing heat removal systems have a significant economic advantage relative to lead and steel material containers for large quantities of spent fuel due to the low costs of material and equipment fabrication.
However, concrete containers with heat removal systems suffer from technical disadvantages. Because the surface material characteristics of concrete include high surface porosity, these containers cannot be lowered directly into the nuclear power plant water pools, without permanently contaminating the pores in the concrete surface and thereby exceeding the codes and standards established for outdoor transfer and storage. A steel and lead storage container must be used as an intermediate transfer containers. This container must be purchased and operated as an intermediate step to move the spent fuel from the water storage pool to the concrete container. The spent fuel elements are loaded into the intermediate transfer containers in a similar manner as the steel and lead material storage containers. Prior to transferring the contents of the transfer containers to the outdoor concrete containers, large quantities of loose radioactive contamination must be chemically removed from the container outer surface. Once decontaminated below the applicable nuclear standards and code levels, the transfer container is moved outdoors and a complex and time consuming transfer is carried out from the transfer container to the concrete shielded container. The empty transfer container is returned to the water pool and the operation is repeated.
A further disadvantage of the concrete containers utilizing heat removal systems is the significant exposure of nuclear power plant personnel to radiation associated with the transfer of spent fuel when conducted outside of the water storage pools and toxic chemicals associated with the chemical decontamination of the steel and lead transfer container.
Yet another disadvantage of conventional concrete containers is that the transfer of spent fuel into concrete containers takes place away from the nuclear power plant, thereby exposing the general public to a risk of significant nuclear radiation.